# Tp 14 specific latent heat (shared)

6 de Dec de 2012

### Tp 14 specific latent heat (shared)

1. A-level Physics Unit G484: The Newtonian World Specific latent heat Thermal physics
2. Internal energy LOs 1. Define internal energy 2. Write a statement of the first law of thermodynamics. 3. Describe two ways in which the internal energy of a body can be increased. 4. How would you identify a body whose internal energy had increased? 5. Is it possible to increase the internal energy of a body without changing its temperature? 6. Energy can be added to a body without raising its temperature. How is this possible? Thermal physics
3. Lesson focus • Specific latent heat Learning objectives At the end of the lesson you will be able to: • describe what is meant by the terms latent heat of fusion and latent heat of vaporisation. Thermal physics
4. Learning outcomes All of you should be able to • explain why a substance has two latent heats; • give a full definition of the latent heat of fusion and of vaporisation; • recall the equation for latent heat; • solve straightforward problems involving latent heat. Most of you will be able to • solve more complex problems involving latent heat. Thermal physics
5. Specific latent heat LOs Definition A specific latent heat of a substance is the amount of energy needed to change the state of _____ of the substance without changing its ________________ . This energy may be absorbed or released depending on the direction of the change of state. Any substance will have two specific latent heats: • latent heat of fusion (Lf ; solid liquid) • latent heat of vaporisation (Lv ; liquid gas) [note spelling] Unit: __________ Thermal physics
6. Specific latent heat LOs Demonstration 1: the latent heat of vaporisation of water We are going to find LV for water by electrical heating. • Set up appropriate word and symbol equations to represent the energy transfer that will take place. electrical energy energy needed to change = supplied to water state of water (liquid to gas) i.e. E = mL where, m – mass of substance L – specific latent heat Thermal physics
7. Specific latent heat . LOs Demonstration 1: the latent heat of vaporisation of water • Sketch a temperature – time graph to show the water being heated towards boiling point. • Describe and explain any changes to the internal energy of the water. • Now continue the graph as boiling starts. • Describe and explain any further changes to the internal energy of the water. Thermal physics
8. Specific latent heat . LOs Demonstration 1: the latent heat of vaporisation of water The accepted value for water = 2.26 x 106 J kg-1 To do 1. Comment on, and explain the difference between this value and our experimental result. 2. Calculate the percentage difference between these values. 3. How can a more accurate determination of LV for water be made? Thermal physics
9. Changing internal energy LOs temperature time To do • On whiteboards sketch a graph showing the temperature of ice as it is steadily heated from -10 °C to 110 °C. • Describe and explain change to the internal energy in each section of the graph. Thermal physics
10. Changing internal energy LOs Figure from Hyperphysics Thermal physics